Tuesday, August 09, 2005

611: Chirality in Snails

There are 22 species in the group, and 5 of them are sinistral, with the rest dextral. The question is whether speciation events can reasonably be traced back to changes in a single gene, whether this diverse assemblage can be explained by occasional mutations in chirality that split off new reproductively isolated groups.

To make a long and somewhat mathematical story short, the answer is no. There have to be other isolating mechanisms present to help out.

One observation is that if you are a newborn dextral snail in a population of sinistrals, you're going to have a much harder time finding a mate than your sinistral cousins. The more common your morphology, the more likely you are to find a compatible mate. This competitive advantage for the most common form will typically drive the population towards a single chirality.

There are, however, conditions under which it is good to be a weirdo. When two species of the same chirality overlap, it will be common for individuals of those two species to mate—which may be fun, but it's fruitless. If one species has a subpopulation with a different chirality, though, they may have an advantage. While they are only able to mate with conspecifics of the same handedness, they won't be wasting time and gametes on members of the other species. This is a phenomenon called character displacement, and could be an additional force for speciation.

In the simpler case where a single population has two chiral variants, though, chirality is insufficient in itself to isolate the two forms. With mathematical modeling, the authors showed that the separation will be incomplete because of gene flow, so the two types will reach an equilibrium, but outside of chance variations, one will not replace the other. The catch is the way maternal effects are delayed in the expression of their phenotype by a generation. That means that a sinistral snail can mate with a sinistral snail, and their progeny may be dextral, and able to breed with the dextral population. Similarly, some of those dextral snails will mate with other dextral snails, and produce progeny which are sinistral. Gene flow is slowed between the two subgroups, but it would require other phenomena, such as geographic separation, to complete the process.

Chirality is how the snail shell twists. Sinistral is to the left, dextral to the right. This is a speciation being studied, evolutionary hypotheses tested, and new ones being generated.